I don't have any solutions, but this might be possible. People have modified the firmware in various makes of cameras, like Sony and JVC. If you google, and search by posts for make or model, "hack" or "modification", English, and "firmware" or "bios" and also search these forums (select search by post option if you get long threads).
The one problem that might be faced, if it is possible, is that the length and type of text for Japanese menus is different from English equivalent, even if it supports English text, it might store the text in a space of memory smaller then the English text can fit, needing the program code to be move, which might not be easily done. Their might be the possibility that the US firmware can be flashed to the Japanese version, but the camera might have differences that will make it not work, crash, or some things not work, or might even fry the camera.
But, there was a guy selling various black mamba models, he seems to have instructions in English, maybe he will know something as it would be in his interests to find a way for people to change to English menus.
(please note, I do not assert he is a good place to buy from, I don't know enough to say that).
http://www.supervideo.com/MXmouse.htm
http://www.supervideo.com/panason.htm
http://www.supervideo.com/GS400.htm
http://www.supervideo.com/MXExtreme.htm